Soil water, also known as rhizic water, is a fundamental component of soil, acting as the largest terrestrial water reservoir on Earth, holding about two-thirds of all fresh water globally. This water is a medium for transporting nutrients, plays a role in soil formation, and influences the exchange of heat and water energy between the land surface and the atmosphere. Maintaining proper soil water levels is important for overall ecosystem health and function.
Forms of Soil Water
Water exists within the soil in several distinct forms, each interacting differently with soil particles and varying in availability to plants. Gravitational water is a free form of water loosely held in the soil, primarily within macropores, which are larger soil spaces. This water drains quickly due to gravity and is generally not available for plant use.
Capillary water is held in the micropores between soil particles. This water forms the soil solution and is retained by capillary action, a force stronger than gravitational attraction in these small pores. It is the most accessible form for plants and their primary water source.
Hygroscopic water forms a thin film around soil particles and is tightly bound by adhesion forces. This water is generally unavailable to plants because it is held too firmly by the soil particles. Soils with a high percentage of clay may contain more hygroscopic water.
Movement of Soil Water
Water movement through the soil involves several interconnected processes, driven by various forces. Infiltration is the initial entry of water from the surface into the soil. After infiltration, water moves downward through the soil layers in a process called percolation, primarily driven by gravity and capillary forces.
As water moves through the soil, it can eventually reach groundwater. Evaporation is the process where water turns into vapor and is lost from the soil surface to the atmosphere. Transpiration is the process by which plants absorb water through their roots and release water vapor into the atmosphere through pores in their leaves.
The movement of water within the soil is governed by differences in water potential, with water moving from areas of higher potential to areas of lower potential. In saturated soils, gravity is the dominant force driving rapid downward movement through larger pores. In unsaturated soils, the attraction of soil surfaces for water, known as matric potential, becomes a major driving force, allowing water to move laterally or upward through capillary action.
Soil Water and Plant Growth
Soil water is essential for plant growth, participating in several physiological processes. It serves as the medium from which plants absorb necessary nutrients, as these are dissolved in the soil solution and transported to the roots. Water also acts as a reactant in photosynthesis, the process by which plants convert carbon dioxide into sugars using light energy.
Adequate soil moisture is also necessary for maintaining turgor, the internal water pressure that keeps plant cells firm and supports the plant’s structure. When plants lose water faster than they can absorb it, cells lose turgor, leading to wilting. Optimal moisture conditions enhance photosynthetic activity, while severe water deficit reduces it.
Two important concepts describe soil water availability for plants: field capacity and the permanent wilting point. Field capacity refers to the maximum amount of water a soil can hold against gravity after excess water has drained away. This represents the ideal moisture condition where water is readily available for plant uptake.
The permanent wilting point is the minimum amount of water in the soil below which a plant cannot recover its turgidity. At this point, the force holding the remaining moisture to soil particles exceeds the plant roots’ ability to absorb it, leading to irreversible wilting. The water held between field capacity and the permanent wilting point is plant-available water.
Factors Influencing Soil Water
Several soil properties and environmental factors directly influence how much water soil can hold and how it behaves. Soil texture, the proportion of sand, silt, and clay particles, directly affects water retention. Sandy soils, with their larger particles and pore spaces, allow water to drain quickly and have a lower water-holding capacity. Conversely, clay soils, composed of fine particles and tiny pores, can hold water more tightly, resulting in higher water retention, though they may have slower drainage. Silty soils have medium-sized particles, offering better water retention than sand and moderate drainage characteristics.
Soil structure, referring to the arrangement of soil particles into aggregates, also impacts porosity and water movement. Well-structured soils with stable aggregates create a network of various pore sizes, allowing for good aeration and drainage while also holding water for plant roots. Compaction, which reduces pore space, decreases porosity and hinders water circulation.
Organic matter content in the soil plays a significant role in increasing water-holding capacity. Organic matter particles have charged surfaces that attract and retain water, acting like a sponge. For instance, a 1% increase in soil organic carbon can lead to a significant increase in water-holding capacity.
External influences like topography and climate also affect soil water. Topography, or the land’s elevation and slope, influences water drainage and erosion patterns. Climate, including temperature and precipitation patterns, determines the overall hydrological and temperature regimes of soils, impacting weathering, leaching, and the development of soil horizons.